The present invention relates generally to black oil heaters and delayed coker heaters used in crude oil refining. More specifically, the present invention discloses a system and method whereby on-line cleaning of black oil heater tubes and delayed coker heater tubes may be effectuated by injecting a high pressure water charge through the tubes during normal process operation so as to prevent tube fouling and heater downtime. The present invention can be used for a variety of applications including, but not limited to, cleaning the tubes of crude heaters, vacuum heaters, visc breaker heaters, delayed coker heaters and any other heaters that have downstream equipment capable of handling the amount of water/steam injected into the heaters.
The present invention can be best understood and appreciated by undertaking a brief review of the crude oil distillation process, and most particularly, the role delayed coking plays within that process.
In its unrefined state, crude oil is of little use. In essence, crude oil is a complex chemical compound consisting of numerous elements and impurities. Such impurities can include, but are not limited to sulfur, oxygen, nitrogen and various metals that must be removed during the refining process.
Refining is the separation and reformation of a complex chemical compound into desired hydrocarbon products. Such product separation is possible as each of the various hydrocarbons comprising crude oil possess an individual boiling point. During refining, or distillation, crude oil feedstock temperature is raised to a point where boiling begins (the “initial boiling point,” or the “IBP”) and continues as the temperature is increased. As the boiling temperature increases, the butane and lighter fraction of crude oil are first distilled. Such distillation begins at the IBP and terminates slightly below 100° F.
The next fraction, or cut, begins slightly under 100° F. and terminates at approximately 220° F. This fraction is represented and referred to as straight run gasoline. Then, beginning at 220° F. and continuing to about 320° F. the Naphtha cut occurs, and is followed by the kerosene and gas/oil cuts, occurring between 320° F. and 400° F., and 450° F. to 800° F., respectively. A term-of-art “residue cut” includes everything boiling above 800° F.
The residue cut possesses comparatively large volumes of heavy liquid hydrocarbons which undergo delayed coking operations to produce various more valuable hydrocarbon fractions and coke. Any suitable delayed coking feedstock can be used as starting material, including vacuum tower bottoms from a crude oil refining process.
Delayed coking is generally carried out by initially heating a liquid feedstock in a coker heater to a coking temperature, often between 875° F. and 950° F. The coker heater includes a coil of multiple coker heater tubes wherein the feedstock is heated before passing to a coker drum. During the coker heater operation, there exists temperature and pressure gradients along the coker heater tubes. Thermal cracking of the heated feedstock occurs primarily in the coking drum to yield mixed volatile hydrocarbon vapors and coke. The vapors are drawn off overhead and introduced to a combination tower wherein hydrocarbon fractions including gases, distillate streams and a heavy gas oil stream are separated and subsequently isolated.
Although thermal cracking of the heated feedstock occurs primarily in the coking drum, there is some premature coking occurring in the radiant coker heater tubes which causes tube fouling. Fouling of the internal walls of the coker heater tubes can cause blockages requiring periodic operation shut-downs to clean the tubes. Modern delayed coking operations present the potential for rapid tube fouling due to increased feed rates and increased concentrations of fouling components in feedstocks such as asphaltenes, inorganics and heavy metals. Usually, it is the capacity of the delayed coker heater which limits the refinery capacity. Therefore, anything that can increase the capacity of the delayed coker heater will result in increased production throughout the refinery. Typically, there is a 3/1 or greater factor capacity increase in crude charge rate to the refinery which corresponds to the increase in delayed coker heater capacity.
The methods that have been used to clean the coker heater tubes' internal walls are on-line spalling, pigging and steam-air decoking. In on-line spalling, the coke is removed from the coker heater tube by varying the steam or condensate flow rate on the fouled tube such that a thermal shock is created that breaks the coke from the tube. The spalling medium transfers the coke particles into the coker heater effluent where it is then collected in the downstream coker drum. An advantage to on-line spalling is that the coker heater is allowed to remain in normal hydrocarbon service because commonly only one pass is spalled at a time while the other passes continue normal operation. However, the cleaning effectiveness of on-line spalling decreases each time it is performed.
For example, a clean coker heater would operate with maximum tube metal temperatures at about 1020° F. and would be allowed to operate until the operating temperature reaches about 1200° F., which would occur in approximately 4 to 6 weeks. Once the operating temperature reaches about 1200° F., on-line spalling is performed which results in the operating temperature being about 1040° F. The coker heater would be allowed to operate until the operating temperature again reaches about 1200° F., which would occur in approximately 3 to 5 weeks. On-line spalling would be performed again which results in the operating temperature being about 1060° F. The coker heater would be allowed to operate until the operating temperature again reaches about 1200° F., which would occur in approximately 2 to 4 weeks. At this time, the entire heater would have to be taken down for a complete cleaning, either by pigging or by steam-air decoking. Thus, on-line spalling only delays, but does not eliminate, the complete cleaning of the coker heater.
In pigging, the coke is removed from the coker heater tube by pushing with water a metal studded foam or plastic “pig” through the heater coil. The metal studded “pig” rotates such that it scrapes the coke off the inside of the coker heater tube. Different sizes and abrasiveness “pigs” are used during this process. The “pigs” are usually pumped through the coker heater several times forward and backward until the overall differential pressure across the tube is restored to its original unfouled condition. An advantage to the “pigging” process is that a significant percentage of the coke is removed from inside the tubes. However, a disadvantage of this process is that the coker heater is required to be taken fully out of service anywhere from 1 to 3 days per heater.
In steam-air decoking, the coke is burned off the coker heater tubes by firing the coker heater in a controlled decoking process by circulating a steam-air mixture at elevated temperatures. The air is used to burn the coke off the tubes, while the steam is used to keep the burning temperatures low such that they do not exceed the maximum allowable tube metal temperatures. An advantage to the steam-air decoking process is that almost all the coke is removed from inside the tubes. However, a disadvantage of this process is that the coker heater is required to be taken fully out of service and heater tubes can be damaged if overheated.
What has been lacking, however, until the present invention, and what the industry long has sought, is an on-line cleaning system and method for black oil heater tubes and delayed coker tubes whereby tube fouling is significantly reduced and/or completely eliminated, thereby allowing the black oil heaters and the delayed coker heaters to remain on-line. The present invention cleans the heater tubes by injecting a high pressure water charge through the tubes during operation so as to prevent tube fouling and heater downtime. The water charge is at a lower temperature than the temperature of the heater tubes. Cleaning occurs by two methods, a scrubbing action caused by the water charge boiling and a shocking action caused by heater tube expanding and contracting due to the temperature differential between the water charge and the heater tube.
It is, therefore, an object of the present invention to disclose and claim an on-line cleaning system and method for black oil heater tubes and for delayed coker heater tubes by injecting a water charge during normal operation.
It is a further object of the instant invention to disclose and claim an on-line cleaning system and method for black oil heater tubes and for delayed coker heater tubes that significantly reduces and/or entirely eliminates tube fouling.
It is still a further object of the present invention to disclose and claim an on-line cleaning system and method for black oil heater tubes and for delayed coker heater tubes that is automated.
It is yet another object of the present invention to disclose and claim an on-line cleaning system and method for black oil heater tubes and for delayed coker heater tubes that cleans by a scrubbing action, caused by the boiling of water, and a shocking action, caused by the temperature difference between the water charge and the heater tube.
It will become apparent to one skilled in the art that the claimed subject matter as a whole, including the structure of the system, and the cooperation of the elements of the system, combine to result in the unexpected advantages and utilities of the present invention. The advantages and objects of the present invention and features of such an on-line cleaning system and a method for black oil heaters and delayed coker heaters will become apparent to those skilled in the art when read in conjunction with the accompanying description, drawing figures, and appended claims.